1. Field of the Invention
This invention relates to a wave reading apparatus, and more particularly to a multiple frequency wave reading apparatus for generating plural signals which have different frequencies for an electronic musical instrument.
2. Description of the Prior Art
An electronic musical instrument of keyboard type must simultaneously generates plural sound signals having different frequencies corresponding to respective keys on the keyboard for polyphonic music. A conventional electronic musical instrument has independent wave generators corresponding to respective keys on the keyboard. Another conventional electronic musical instrument has fewer wave generators than the number of the keys. A generator assigner scans the keyboard and sends a note code and octave code to the wave generator so as to generate a wave signal having the frequency of the note and the octave of a depressed key. The number of wave generators is usually eight to ten, corresponding to the number of human fingers. Still another conventional electronic musical instrument has one wave generator. The wave generator generates plural wave signals in a time-multiplexed operation.
When the wave generators generate the wave signals in the form of a digital code, the generated digital wave samples must be converted to an analog form by a digital-to-analog converter (DAC). The first conventional instrument of the above needs as many DACs as the number of keys. The second conventional instrument needs eight to ten DACs. The third conventional instrument may need only one DAC, but the plural wave signals must be summed in digital form before conversion. Since eight to ten wave signal data must be accumulated at once, a very high speed full adder is necessary. The summed data become larger than each separate data. The bit length of the DAC increases by three to four bits. Accordingly, an expensive DAC must be used. The sampling frequency of the eight to ten wave signals must coincide with each other. This is difficult limitation for a musical instrument, because frequencies of the 12 notes in an octave are different from each other. The ratio of the sampling frequency to the fundamental frequency of wave signal cannot be an integer or a simple fractional number. To solve this problem, the sampling frequency must be very high frequency or a calculation of a complex interpolation between two succeeding wave samples must be executed.